1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using an electrophotographic process, and more particularly to an image forming apparatus and an image forming method for transferring and fixing, after a toner image formed on an image carrier is transferred onto an intermediate transfer member, the toner image on the intermediate transfer member onto a recording medium to obtain a recorded image.
2. Description of the Related Art
As a conventional image forming apparatus, there has been known an image forming apparatus in which toner on an image carrier is primarily transferred onto an intermediate transfer member having a parting property and a toner image on the intermediate transfer member is melted on a recording medium by heating and pressing means for fixing simultaneously with secondary transfer. As this heating and pressing means, a heating roller and a pressing roller which are brought into pressure contact through the intermediate transfer member are known. Toner on the intermediate transfer member is melted at a pressure-contact portion between the two by the heating roll to cause the toner to percolate through the recording medium, and the recording medium is separated from the intermediate transfer member by taking advantage of the parting effect of the intermediate transfer member.
In such an image forming apparatus, in order to transfer a toner image on the intermediate transfer member onto a recording medium and fix it at the same time, there are various severe conditions, and it is considerably difficult to attain transfer and fixing of a good toner image. To this end, as means for improving the transfer and fixing of toner images, there have been proposed techniques described in, for example, Japanese Published Examined Patent Application Nos. Sho 46-41679 (1971), 57-20632 (1982), 58-36341 (1983), and 64-1027 (1989),and Japanese Published Unexamined Patent Application No. Sho 57-163264 (1982).
The technique described in the Japanese Published Examined Patent Application No. sho 46-41679 is to heat a recording medium without heating a toner image on an intermediate transfer member to melt the toner image by the heat of the recording medium for transferring and fixing on the recording medium.
The technique described in the Japanese Published Examined Patent Application Nos. sho 57-20632 and 58-36341 are to selectively heat the toner on an intermediate transfer member to its fusion temperature by radiation heating means without heating the recording medium, and to bring the toner image on the intermediate transfer member into contact with the recording medium for transferring and fixing.
The technique described in the Japanese Published Unexamined Patent Application No. sho 57-163264 is to previously heat the intermediate transfer member and the toner image transferred thereon, and to place the two in close contact with each other in a state in which the recording medium has been heated for transferring and fixing the toner image on the recording medium.
The technique described in the Japanese Published Examined Patent Application No. Sho 64-1027 is to preliminarily heat toner at this side of a nip portion (transfer fixing area) in which the toner image on the intermediate transfer member is urged against the recording medium. More specifically, a belt-like intermediate transfer member is wound around the heating roll by 90.degree. or more to preliminarily heat the toner by the utilization of the heat of the heating roll at this side of the nip portion with the recording medium for raising the toner temperature to near the fusion temperature of the toner. Thereafter, in the nip portion, the toner is further heated and fused to transfer the toner image onto the recording medium and fix it.
The conventional technique as described above, however, has the following problems.
The technique described in the Japanese Published Examined Patent Application No. Sho 46-41679 is desirable in terms of such points that the intermediate transfer member is not likely to be overheated, but the image carrier can be prevented from being thermally affected, but the utilization of heat is low, and a considerably large amount of thermal energy must be applied to a recording medium consisting of ordinary sheet. Further, in the case of transferring and fixing a toner image at high speeds, it is necessary to further apply a large amount of thermal energy to the recording medium, and as a result, the technique has a disadvantage that the consumption energy increases and when the recording medium is not smoothly fed, but jam is caused, combustion is very likely to be caused.
Also, in the technique described in Japanese Published Unexamined Patent Application Nos. Sho 49-78559 and 50-107936, this will be inferior to conductive heating means such as a heating roll in actual thermal efficiency because radiation heating is used as means for selectively heating only toner. Also, since most of thermal energy absorbed by toner is conducted to the intermediate transfer member at lower temperatures while it shifts from a radiation heating area to a transfer fixing area in which it is brought into contact with the recording medium, there is a disadvantage that the toner will have to be heated that much in excess in advance, thus lowering the total thermal energy efficiency. Moreover, since the radiation heating means is used, a paper sheets is likely to be burnt when jam occurs.
On the other hand, the technique described in the Japanese Published Unexamined Patent Application No. Sho 57-163264 has an advantage that the temperature of an intermediate transfer member can be set to be low because the three parties of the intermediate transfer member, toner and recording medium are heated together. Also, there is little heat flow between a toner image on the intermediate transfer member and the recording medium at the pressure-contact portion, the deteriorated fluidity of the toner is low accordingly, and
the toner sufficiently percolates through the recording medium to be transferred from the intermediate transfer member. Since, however, the toner temperature when separated from the intermediate transfer member is higher than the toner softening point temperature and the toner is in a fluid state, there is a tendency that the toner is scattered and is easily offset to the intermediate transfer member. Also, since it is necessary to heat the three parties of the intermediate transfer member, toner and recording medium together, the total consumption energy will become great. In addition, there is a problem that the heat is conducted to the image carrier side by means of the rotational movement of the intermediate transfer member heated by the heating roll and the temperature in its vicinity rises to cause obstruction to the charging function. If an attempt is made following such a mechanism to prevent the heat of the intermediate transfer member from being conducted to the image carrier side, a considerably large cooling equipment will be required to increase the cost of the apparatus.
In the technique described in the Japanese Published Examined Patent Application No. Sho 64-1027, although it is possible to make the set temperature of the heating roll low since the toner is preliminarily heated at this side of the nip portion (transfer fixing area), the total consumption energy will become great in the same manner as the above-described conventional examples because the toner and recording medium are re-heated in the transfer fixing nip portion.